gavanic isolating transformer

[Sea Dog]

Its not just marinas

 

Well, no, but it was his regular discovery of 5v in marinas that Loddon was posting about (and I quoted), hence what I was asking him questions about. It's also arguably the most common/pertinent to most readers, and there's direct read-across too, eh?

[GUMPY]

As DC says its not just marinas.

There are many causes of voltage on the Earth/Neutral far to many to go into in detail here.

In one marina I sat and watched the voltage creep up as people got up and switched things on so yes it can be a cumulative problem.

In another marina (I was told this by a relable source) the voltage appeared when the indusrial unit on the site arrived for work.

It can also be down to bad earth at the substation.

As I said many reasons.

If you are worried about it get your meter out and measure your own marina.

I am not going to name marinas as that could leave me open to legal action.

 

Oh and it was up to 5v, as to how do I discover it I am a picky git when it comes to work and I check everything even if its not directly part of my remit.

 

J

[Keeping Up]

It is worth noting that GIs with led status indicators generally give twice as much protection against such voltages, because they need to use twice as many diodes to provide the voltage needed to drive the LEDs. Also of course you can tell from the LEDs if too high a voltage exists because the LEDs would glow.

[Neil Smith]

So if I go for a GI what amp one should I buy, 58 foot boat 16 amp shore line 3kva combi.

 

Neil

[nicknorman]

There is no "right answer" since the GI doesn't normally pass any power, but 70A seems a common size.

[rogeriko]

As far as voltage on the earth wire is concerned dont forget there is no neutral wire returning to the generators on the National Grid. The return neutral is done through the ground connection, therefore ALL the domestic power in the UK goes to ground in order to complete the circuit back to the nuclear power station or wherever it came from. No wonder your boat is corroding!!! Your metal boat in dirty water is probably the best earth point for the whole town not just the marina. Make sure the marina earth is nowhere near your boat.

[jonesthenuke]

As far as voltage on the earth wire is concerned dont forget there is no neutral wire returning to the generators on the National Grid. The return neutral is done through the ground connection, therefore ALL the domestic power in the UK goes to ground in order to complete the circuit back to the nuclear power station or wherever it came from. No wonder your boat is corroding!!! Your metal boat in dirty water is probably the best earth point for the whole town not just the marina. Make sure the marina earth is nowhere near your boat.

This is misleading and is going to create unnecessary concern. The power supplies are three phase and the overall current (sum of the phases) will be near zero. Also the supplies have multiple transformers en route to the end user. In normal conditions there is no return current flow via earth. This is the simple explanation, so yes there are exceptions. The key issue is how the supply is maintained with neutral near to earth voltage at point of use (i.e. in the marina). As this is something the end user cannot control, consequently I have opted for an isolating Tx.

[smileypete]

So if I go for a GI what amp one should I buy, 58 foot boat 16 amp shore line 3kva combi.

 

Neil

 

Something like the Safeshore 70 amp one with built in or remote status monitor.

 

cheers, Pete.

~smpt~

[markgregoryuk2000]

Unless a GI uses stupidly tiny diodes (wire ended) it would always fail short rather than open in which case you will be safe although without the isolation protection for the boat. I would consider any stud diode as very very unlikely to go open ( it might corrode away I suppose), even a 45A stud will take 500A or more in fault conditions without going open.

 

The 5V mentioned above, is this from neutral to earth or from mains earth to another separate earth point or across a GI. Increase as load increases suggests neutral to earth

[GUMPY]

I have seen both, voltage between N and E and between E and ground.

Edited January 13, 2015 by Loddon

[markgregoryuk2000]

Ouch.

 

5V earth to ground is beyond the isolation of most (if not all) Galvanic Isolators.

 

5V Neutral to Earth is not a big worry, just cable drop due to load probably.

[John Williamson 1955]

Have you identified anywhere that will supply you with a 32A shore supply?

Maybe not on a temporary public mooring, but all the bollards here have 32 Amps per berth available, and some have at least 40 available on a 64 Amp socket. It is, however a council approved and CRT owned residential mooring site that's just had all the installations updated.

[forsberg]

Listing some pros and cons.

General:
In a marina or near boats/houses/workshop etc, it might not help if just you fix your boat.
But in a marina at least it should be possible to make it mandatory to at least have a GI.

 

Nothing at all:
Cheap but puts a lot of trust in the quality of the Power bollard earth quality.

 

Sacrificial anodes:
YES!

 

Power bollard properly earthed:
Check that the nearest groundplate is in working order and prefably below ground water level.
Giving the Power bollard its own ground plate?
Good way to go about it at first thought, except that if there are boats/houses/workshop etc
about with poor earthings you may actually make things worse, much worse.

 

Earthing the sacrificial anode to the boat earth point:
Probably a good thing?

 

Galvanic Isolator:
Not very expensive. Protects against against small earth current flows.
It's also good if it has an indicator so you can tell if the current is too large.
The more diodes in the unit the better, ie 2 diods can stopp .5-.7V, 4 will stopp 1-1.4V etc.
Anything over this will pass thrue slightly reduced. See "Power bollard"
If faulty probably stops protecting against corrosion but no personal hazard.
The opposite may occur.
So checking it once in a while is not a bad thing especcialy after lightning.
Does not protect against current passing thrue the shoreline in the N and F leads to the hull if you have any Residual-Current leaks.

Isolating Transformer:
Can be connected in some diffrent configurations each with diffrent pros and cons.
Generally it does nothing as long as your equipment dont have any leaks to earth.
But as there usually are some it will stopp current passing thrue the shoreline in the N and F leads to the hull.
It will also lower the risk for corroision from internal current leaks, but not remove it compleatly.
NOTE: If not also fitted with a Galvanic isolator (interal or external) it does not protect from earth currents.
Industrial types will not have a galvanic isolater, others may but please check.
NOTE2: The landline RCD now only protects the extension cable to the boat.

 

Some may argue that you do not need a connection between the landline earth and the boats earth and thus not need a GI
when a IT is fitted.
This may be correct in theory but not in practice.
For this configuration the primary thing to note is that your boat may not be earthed enough to allow fuses
to trigger, to ensure this a copper plate 20*20cm minimum should be fitted on the hull below the waterline
and connedted to the boat earthpoint. Some may suggest that the sacrifical anodes and propeller will substitute for this
but thats not something you leave to chance.
All in all, DONT! http://www.smartgauge.co.uk/earthing.html

 

Residual-Current Device (RCD):

If the one in the power bollard fails to trigger it never hurts to have one in the boat too.
As noted above you would like to have a RCD after the IT as well.

If you just have a GI, fitting 10mA ones tells you if the current leaks to the hull exceeds 10mA.
A 30 mA type will protect human life (barley) and tell you if the current leaks to the hull exceeds 30mA.
A 10 ma will protect life and and tell you if the current leaks to the hull exceeds 10mA.
They also reduce risk for electrical fires.
Again some may argue that a 10mA one will trigger all the time.
Hmm yes possibly but the point is that in that case something is wrong, and they dont know how to fix it.
If you consider this to be a possible problem you could fit one RCD for each major appliance and one for sockets.
Or you could fit a 30ma one for major appliances and a 10ma for sockets.
Calorifier heaters, washing machines and electric stoves are good candidates for developing earth faults.

 

 

 

[nicknorman]

 

 

Galvanic Isolator:

...

It's also good if it has an indicator so you can tell if the current is too large.

 

 

 

I think you mean voltage?

 

Isolating Transformer:

Can be connected in some diffrent configurations each with diffrent pros and cons.

Generally it does nothing as long as your equipment dont have any leaks to earth.

But as there usually are some it will stopp current passing thrue the shoreline in the N and F leads to the hull.

It will also lower the risk for corroision from internal current leaks, but not remove it compleatly.

NOTE: If not also fitted with a Galvanic isolator (interal or external) it does not protect from earth currents.

Industrial types will not have a galvanic isolater, others may but please check.

NOTE2: The landline RCD now only protects the extension cable to the boat.

 

Some may argue that you do not need a connection between the landline earth and the boats earth and thus not need a GI

when a IT is fitted.

This may be correct in theory but not in practice.

For this configuration the primary thing to note is that your boat may not be earthed enough to allow fuses

to trigger, to ensure this a copper plate 20*20cm minimum should be fitted on the hull below the waterline

and connedted to the boat earthpoint. Some may suggest that the sacrifical anodes and propeller will substitute for this

but thats not something you leave to chance.

All in all, DONT! http://www.smartgauge.co.uk/earthing.html

 

 

 

So if I understand you correctly, you are proposing that an IT should be installed, with the shore earth connected to the boat-side earth via a GI. If so, could you explain what the point of having an IT in the first place is?

 

The generally perceived wisdom is that there is no connection between shore earth and boat earth, but the screen / earth connection on the IT is connected to the neutral on the secondary (output). As per your reference to the smartgauge website!

Edited January 15, 2015 by nicknorman

[jonesthenuke]

Taking some of the points by Forsberg above whilst its a good discussion I would challenge some points (I am no expert so would welcome comments).

 

I agree the power bollard should have an effective earth, but what reasonable check can most people make of this? I suspect no marina will allow to check their equipment in any intrusive way. This is one of the reasons I have fitted an IT.

 

On the issue of earthing I agree we need to be careful and consider fault situations, however with regard to the sentence:-

 

"For this configuration the primary thing to note is that your boat may not be earthed enough to allow fuses

to trigger"

 

What is this based on? For most standard boats, the earthing is not limited to the prop and the anodes as suggested as the baseplate is often not blacked. This would provide a very large earthing electrode surface even when rusty and with biological growth. I would thus challenge the claim that fuses will not be effective (and assume this also means circuit breakers). It would be interesting to measure the earth impedance of a boat to a nearby test electrode (perhaps to the steelwork supports of the marina decking adjacent to my boat).

 

The same applies to the risk of the hull being at 230V in relation to local earth. This would require very effective isolation of the boat from the water in combination with wholly ineffective earthing of the incoming supply' neutral or failure to operate of the RCD. The isolation of the hull may be pertinent to GRP hulls etc, but is very much less likely to be an issue with a steel hull.

 

So returning to my first point, I see one of the key issues being the design and proper maintenance of the supply on the land side. GIs will not cope well with a defective supply and safety protection is based on effective earthing (at the bollard) and a good tie between neutral and earth at an appropriate point in the supply system plus an effective operable RCD. So what standard should apply to Marina supplies and do Marinas comply? I do not have the answers to this but if I get time I will browse the latest IEE regs (and I am sure someone on here will know!)

The standard is section 709 of the current BS 7671. I understand this requires annual inspections. Will try and get a copy.

A google search gives the following:-

Edited to change the last sentences.

Edited January 15, 2015 by jonesthenuke

[forsberg]

Hi Nicknorman.

Well I thought about that when writing , and yes it is volt that is measured but with the intention to find out if there is a current.

You can have volt without current but not current without volt. As measuring the current directly would be almost impossible we'll have to do with second best.

The point of the IT i did describe, it will stop any leak currents in your equipment, now wait... I wasnt clear enough there.

It will stop the leak currents in your equipment passing out thrue the hull, and also removes the possibility that they get amplified by leaks from other boats etc.

So corrosion due to that will be stopped.

I think we read the Smartgauge page in diffrent ways. But yes making your own earth is widely used and the risk is probably not high on a steel hull with a non painted bottom plate.

But words like probably or most are not something you want to use in regards to electrics, using them too often might end up with someone dead.

 

 

Hi Jonesthenuke.

Well it's always best to address a problem at it's source and a Marina would be very daft to let the customers boats to corrode.

You dont actually need to be intrusive, just take a hose and water the base of the bollard (if on land) if your GI reading improves then there are room for improvments.

You can do the same for the nearest ground plate, but it may take a few days to water down.

People do tend to over estimate the conductivity of fresh water.

[nicknorman]

You can have volt without current but not current without volt. As measuring the current directly would be almost impossible we'll have to do with second best.

.

Well just to be pedantic you can have current without voltage - in a superconductor! You can measure current without voltage using its magnetic field (Hall effect etc).

[jonesthenuke]

Hi Nicknorman.

Well I thought about that when writing , and yes it is volt that is measured but with the intention to find out if there is a current.

You can have volt without current but not current without volt. As measuring the current directly would be almost impossible we'll have to do with second best.

The point of the IT i did describe, it will stop any leak currents in your equipment, now wait... I wasnt clear enough there.

It will stop the leak currents in your equipment passing out thrue the hull, and also removes the possibility that they get amplified by leaks from other boats etc.

So corrosion due to that will be stopped.

I think we read the Smartgauge page in diffrent ways. But yes making your own earth is widely used and the risk is probably not high on a steel hull with a non painted bottom plate.

But words like probably or most are not something you want to use in regards to electrics, using them too often might end up with someone dead.

 

 

Hi Jonesthenuke.

Well it's always best to address a problem at it's source and a Marina would be very daft to let the customers boats to corrode.

You dont actually need to be intrusive, just take a hose and water the base of the bollard (if on land) if your GI reading improves then there are room for improvments.

You can do the same for the nearest ground plate, but it may take a few days to water down.

People do tend to over estimate the conductivity of fresh water.

Point taken, but my bollard is on wooden decking well above the water and its plastic, so I will not see much from watering that. This applies to many marinas. As for the earth plate, how would I find it so that I can water it? This supports the point I have been trying to make; if the system is well designed and properly maintained then all will probably be OK. However, its simply beyond the average boater to undertake appropriate tests for themselves.

 

I remain of the view that isolating transformers are the lowest risk solution, and one worth considering unless you are very sure of the quality of the electrical supply.

[smileypete]

Why not just fit a GI with status monitor?

 

In the unlikely event the boat is moored on one of the 0.1% of moorings with serious earthing problems, THEN worry about plan B which may be fitting an isolation transformer.

 

You could argue that 0.1% of moorings may have a defective RCD shore side, which could make a steel narrowboat with IT on board and hull isolated from shore earth more dangerous than a GI.

 

cheers, Pete.

~smpt~

Edited January 16, 2015 by smileypete

[Keeping Up]

Why not just fit a GI with status monitor?

 

In the unlikely event the boat is moored on one of the 0.1% of moorings with serious earthing problems, THEN worry about plan B which may be fitting an isolation transformer.

 

You could argue that 0.1% of moorings may have a defective RCD shore side, which could make a steel narrowboat with IT on board and hull isolated from shore earth more dangerous than a GI.

 

cheers, Pete.

~smpt~

Seconded

[GUMPY]

Why not just fit a GI with status monitor?

 

In the unlikely event the boat is moored on one of the 0.1% of moorings with serious earthing problems, THEN worry about plan B which may be fitting an isolation transformer.

 

You could argue that 0.1% of moorings may have a defective RCD shore side, which could make a steel narrowboat with IT on board and hull isolated from shore earth more dangerous than a GI.

 

cheers, Pete.

~smpt~

Interesting

Where do you get the figure of 0.1% have you been round checking marinas and moorings?

[EEng]

Why not just fit a GI with status monitor?

 

In the unlikely event the boat is moored on one of the 0.1% of moorings with serious earthing problems, THEN worry about plan B which may be fitting an isolation transformer.

 

You could argue that 0.1% of moorings may have a defective RCD shore side, which could make a steel narrowboat with IT on board and hull isolated from shore earth more dangerous than a GI.

 

cheers, Pete.

~smpt~

 

You shouldn't rely on a shore RCD - have your own and make sure they are tested every 6/12 months - RCDs are prone to failure (they get lazy and either die or take too long to trip) and if your on a boat then its more dangerous with all the water around you than in a house on land.

 

You could also ask if the marina has had an EICR done and have all the RCDs been tested, marinas have a special section in BS7671 (IET Wiring Regulations)

 

Perhaps you could explain if the supply is IT then how an RCD failure would be more dangerous (dangerous to what) ?

[rogeriko]

Surely the whole point of using an isolation transformer is to seperate the live to ground connection. In other words the live output of the transformer is only at a potential of 230v above the neutral output of the transformer NOT to ground. In europe (different regs) I used to install outdoor lighting through a 230v transformer and the neutral is never connected to ground. There is no RCD after the transformer. This is how it is done europe wide. You cannot get a shock from the live wire because there is no ground to complete the circuit, only if you touch live and neutral at the same time. I used to work on live radio and tv chassis, remember those, where one side of the mains went directly to the chassis. Anyway we used to use isolating transformers and then could quite safely touch all metal parts live or not. I know RCD's are the norm (regs) in the UK but having one after the transformer and connecting neutral to ground kind of defeats the object.

Edited January 16, 2015 by rogeriko

[EEng]

Surely the whole point of using an isolation transformer is to seperate the live to ground connection. In other words the live output of the transformer is only at a potential of 230v above the neutral output of the transformer NOT to ground. In europe (different regs) I used to install outdoor lighting through a 230v transformer and the neutral is never connected to ground. There is no RCD after the transformer. This is how it is done europe wide. You cannot get a shock from the live wire because there is no ground to complete the circuit, only if you touch live and neutral at the same time. I used to work on live radio and tv chassis, remember those, where one side of the mains went directly to the chassis. Anyway we used to use isolating transformers and then could quite safely touch all metal parts live or not. I know RCD's are the norm (regs) in the UK but having one after the transformer and connecting neutral to ground kind of defeats the object.

 

it would depend how the E and N are wired on the Isolating Transformer Circuit - There is more than one way of doing it

[rogeriko]

I mean the earth dosn't go to the transformer at all (on the boat side) the live and neutral from the transformer go to the consumer unit and then to the outlets. The ground from the outlets is connected to the hull but not to the neutral. That way both wires L/N from the transformer are floating above ground and no current will flow through you to ground in the event of a fault.